For example, a spark plug disclosed in Patent Document 1 has been known as a spark plug whose radial direction is taken as a direction of discharge (hereinafter also called “plug of lateral discharge type”).
(JP-A-2002-83662, JP-B-3497015, and the like)
In order to suppress a flame-extinguishing action on flame kernel, as one kind of related-art spark plug, there is provided a spark plug in which a noble metal tip is welded to each of a leading end of a center electrode and a distal end of a ground electrode. When a noble metal tip is welded to an end of an electrode, if a difference between the diameter of the electrode and the diameter of the noble metal tip is large, it is difficult to perform balanced welding, which may deteriorate a welding strength. For this reason, in order to make the diameter of the center electrode close to the diameter of the noble metal tip, there is often adopted a structure in which the diameter of the center electrode is reduced toward a leading end thereof. However, since an angular part is formed at a starting point where the diameter of the center electrode is reduced, there is a problem, in the related-art plug of the lateral discharge type, of occurrence of a case where electric discharge occurs between the angular part formed at the starting point where the diameter of the center electrode is reduced and the ground electrode.
The present invention has an object thereof to provide a technique for a plug of lateral discharge type capable of suppressing occurrence of electric discharge in portions other than a normal discharge path.